Golf clubs of all types generally have a golf club head that has a sole portion. The sole portion of the golf club head interacts with the ground on most golf shots. Such interaction with the ground is commonly known as turf interaction.
It is generally desirable to control the degree of turf interaction, for example by minimizing the amount of interaction between the golf club head and the ground during a golf shot, particularly for low-lofted clubs. For example, in a case where a golf ball has a “bad” lie, i.e. where the turf is long, the turf will impede the golf club head during the golf shot, thereby reducing a club head speed of the golf club head, which in turn reduces shot distance. Further, the turf may deflect the club head during the course of a swing, negatively affecting the orientation of a striking face of the club head at the point of impact. In other cases, the golf ball may be on an uneven lie, a turf-less lie, a hard-pan lie, a wet lie, or even a good lie in a fairway (where the turf is short). Any of these varying lies could have a negative effect on an outcome of a golf shot by imparting drag on the golf club head or deflecting the golf club head as it approaches a golf ball during the golfer's swing. However, the effect of turf interaction, as well as its desirability, varies from one club head to another within a correlated set, particularly with regard to increasing loft angle.
However, configuring sole structure to control turf interaction may have unintended consequences. For example, configuring a sole portion of club head to control turf interaction, inter alia, decreases an amount of discretionary mass that could otherwise be positioned to improve mass properties of the club head, e.g. the magnitudes of moments of inertia about the center of gravity and/or the orientation and magnitude of the principle axes of inertia passing through the center of gravity. Also, by generally removing mass from a sole portion of a club head, e.g. forming a keel-shaped sole portion, a center of gravity height is increased, adversely affecting shot trajectory.